Approaching this from a Catholic perspective, I'd like to ask believers how they came to know that they were called to join the religious life (as a brother or nun), as opposed to the other vocations (priesthood, marriage, single life etc). Rephrasing the same question, what should a believer look for when discerning whether they are called to the vocation of religious life?
Here are some resources:
- Vocation to the Religious State by St. Alphonsus de Liguori
- Vocations Explained: Matrimony, Virginity, the Religious State and the Priesthood by A Vincentian Father.
- Vocations by Rev. William Doyle, S.J.
- Religious Vocation: An Unnecessary Mystery by Fr. Richard Butler, O.P.
See also Pope Pius XII's apostolic constitution Sede Sapientiæ on vocations to the religious life says (part II.) that two necessary conditions must be met when discerning a vocation: that one is called by God and called by the Church.
I can't answer directly from a Catholic point of view, but the identification of a calling to the priesthood in the Anglican church probably has some similarities. I also know people who have been called to religious communities.
The key thing is that the identification of such a calling is done by the community. Thus you don't show up at a theological college (or presumably a monastry) and ask to be admitted. Instead it is done by approaching your own parish priest, who will also talk to other people who know you, and probably instruct you to do the same. At later stages a Bishop will be involve, and also a group of people who have special skills in discerning such callings.
Discernment of the calling has usually taken many years. There have been retreats and visits to religious communities, and discussion and prayer with the people in those communities, as well as other people.
Why? This will be the question asked most often. "Why do you want to be a...?" And related: "What can you do as an ordained person that you cannot do as a lay person?"
The theology of 'calling' is pretty low in my Diocese (Episcopal). That is to say, it's less of a calling and more of an offering. An offering of where one is and what gifts one has and how God can make use of what you bring.
The saying is trustworthy: If anyone aspires to the office of overseer, he desires a noble task. Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach, not a drunkard, not violent but gentle, not quarrelsome, not a lover of money. He must manage his own household well, with all dignity keeping his children submissive, for if someone does not know how to manage his own household, how will he care for God's church? (1 Timothy 3:1-13)